1. Field of the Invention
The present invention is directed to a target for the ultrasonic location of an article implanted beneath the skin.
2. Brief Description of the Prior Art
Objects for implantation beneath the skin are well known in the art. Among such articles for implantations are tissue expander devices. Such tissue expander devices are implanted beneath the skin thereafter to be inflated to stretch the overlying tissue to prepare, for example, a skin flap for grafting. More commonly, tissue expander devices are implanted beneath the skin and slowly inflated to create a pocket into which a prosthetic device may be permanently placed, such as a mammary implant. Such tissue expander devices require slow inflation. This is accomplished by the co-implantation of an injection port with a fill reservoir connected to the interior of the tissue expander. Some prior art fill ports are domed to enable their location beneath the skin by means of palpation. Once located, the fill port reservoir is accessed by means of the transdermal placement of a needle into a self-sealing septum on the injection port. The subsequent injection of fluid into the port inflates the expander device. Since it is necessary to pierce the septum (which septum is self-sealing to the track of a needle when the needle is removed), it is fundamental that means for accurately locating the septum beneath the skin are available.
To enable the facile location of deeply implanted injection ports, Sampson, et al., U.S. Pat. No. 4,222,374, describe a device for locating the septum of the injection port by means of a magnetic indicator. Sampson's septum locating apparatus comprises means mounted in an implantable device for producing an energy pattern that emanates from the patient's body. The pattern is shaped so as to "target" the device's septum that underlies the patient's skin. In one embodiment of the invention the energy pattern-producing means is one or more permanent magnets which are shaped and/or arranged relative to the septum so as to generate a magnetic field pattern external to the patient's body which indicates the septum's location.
In general, Sampson teaches the placement within the "target" of a radiation source; the radiation source of preference being a magnet. Other radiation sources might include radioactive elements. The only constraint being that the energy pattern production means comprises one or more sources of corpuscular radiation mounted in the implant device in juxtaposition with its septum so that the radiation pattern emanating from the patient's body provides an indication of the septum location.
Sampson, et al., in their preferred embodiment, include a small hand held detector that is responsive to the magnetic energy pattern produced by the septum designating source incorporated into the implanted device. When the detector is moved along the patient's body over or adjacent to the general vicinity of the implanted device, the detector responds to the energy pattern by the motion of a gimbled bar magnet thereby producing a visible indication of the location of the septum relative to the detector. Thus, following these indications the physician manipulating the detector can steer the detector until a reticule on the detector directly overlies the implanted septum. Then using the reticule as an aiming point, the physician can mark the patient's skin and be assured that the implanted septum is located directly underneath that mark.
There are two problems associated with the use of magnets in juxtaposition to the septum. The first is that most such magnetic materials capable of generating a magnetic field are radiopaque. Thus, tumors and the like adjacent to a radiopaque radiation source would not be apparent under x-ray. Increasingly, whole body nuclear magnetic resonance spectroscopy (NMR) is gaining favor in diagnostic laboratories. The NMR diagnostic technique requires the use of magnetic fields penetrating the body. The accuracy of NMR depends upon the magnetic field strength at a particular point in the body. The external field imposed by the nuclear magnetic resonance device can be altered locally by the presence of the magnet within an injection port rendering diagnostic imaging of tissues close to the magnet by NMR difficult, if not impossible. Further, if an implanted target should have its integrity breached, the release of ferromagnetic material or the exposure of the body to ferromagnetic material comprising the target might have deleterious results. For these reasons it is desirable to have a non-magnetic target. It would be further desirable to have a targeting signature which is easily distinguished from surrounding tissue yet is not radiopaque.